Pnictide-based semiconductors include the Group IIB/VA semiconductors. Zinc phosphide (Zn3P2) is one kind of Group IIB/VA semiconductor. Zinc phosphide and similar pnictide-based semiconductor materials have significant potential as photoactive absorbers in thin film photovoltaic devices. Zinc phosphide, for example, has a reported direct band gap of 1.5 eV, high light absorbance in the visible region (e.g., greater than 104 to 105 cm−1), and long minority carrier diffusion lengths (about 5 to about 10 μm). This would permit high current collection efficiency. Also, materials such as Zn and P are abundant and low cost.
Zinc phosphide is known to be either p-type or n-type. To date, it has been much easier to fabricate p-type zinc phosphide. Preparing n-type zinc phosphide, particularly using methodologies suitable for the industrial scale, remains challenging. This has confounded the fabrication of p-n homojunctions based upon zinc phosphide. Consequently, solar cells using zinc phosphide most commonly are constructed with Mg Schottky contacts or p/n heterojunctions. Exemplary photovoltaic devices include those incorporating Schottky contacts based upon p-Zn3P2/Mg and have exhibited about 5.9% efficiency for solar energy conversion. The efficiency of such diodes theoretically limits open circuit voltage to about 0.5 volts due to the about 0.8 eV barrier height obtained for junctions comprising Zn3P2 and metals such as Mg.
Improved efficiency and open circuit voltage would be expected, though, from p/n homojunction cells for which the junction is formed by contiguous regions of the same semiconductor material having p and n type conductivity, respectively. One exemplary advantage of a p/n homojunction would be a minimization of discontinuity in the energy band structure while the gross composition remains the same. Also, indices of refraction of the adjacent p/n material would match, minimizing reflection losses. Also, the coefficients of thermal expansion would be matched to minimize potential delamination risks.
Some investigators have suggested that a p/n homojunction can form in situ when a layer of p-type zinc phosphide is heated while in contact with magnesium. See, e.g., U.S. Pat. No. 4,342,879. Other investigators have prepared n-type zinc phosphide using molecular beam epitaxy. Other approaches to make n-type zinc phosphide also have been attempted. However, such approaches generally yield devices with poor photovoltaic behavior, if any, due at least in part to poor film quality, lack of control over film stoichiometry, and/or lack of control over formation of high quality p/n junctions.
Much research and development effort is focused upon improving the electronic performance of optoelectronic devices, particularly photovoltaic devices that incorporate pnictide-based semiconductors. One challenge involves the surface quality of the pnictide film as deposited. Often, the surface quality of such surfaces is inadequate for further device formation due to issues such as roughness, electronic defects, crystalline structure defects, contamination, and the like. Accordingly, one or more kinds of treatments are practiced in order to improve the surface quality. For example, mechanical polishing has the benefit of planarizing rough surfaces, but tends to damage surface crystal structures. Hydrogen plasma treatments clean impurities, but damage surface crystal structure. Conventional etching techniques using etching compositions such as Br2 in methanol have been used to remove polishing and plasma damage, native oxides, and other impurities such as adventitious carbon, but then the resultant surface is of low electronic quality. Consequently, strategies for providing pnictide films with improved electronic characteristics are still needed.